Septic tank systems, also referred to as on-site sewage treatment and disposal systems (OSTDS), are frequently used for wastewater treatment and disposal. When properly designed, installed and maintained, an OSTDS can be effective in removing organic matter, bacteria and nutrients from wastewater. However, certain conditions, such as proximity to surface waters, unsuitable soils, high water tables or improper maintenance or use can interfere with or prevent proper functioning of an OSTDS. When such conditions exist, the level of treatment may be insufficient to protect water quality in nearby receiving bodies of water. Wastewater effluents, although meeting all permitted water quality criteria, can also result in deleterious water quality effects.
Many conventional sampling methods for assessing wastewater contamination rely on determining concentrations of fecal coliform bacteria. Although high concentrations of fecal coliform bacteria are found in human sewage, numerous non-human sources of fecal coliform are present in urbanized watersheds, with many animals producing far greater daily fecal coliform counts than humans. Other water quality parameters often used to test for the presence of septic effluents (e.g., total organic carbon, total nitrogen, total phosphorous, or the ratio of fecal coliform to fecal streptococcus colonies) also have non-anthropogenic sources. As a result, the presence of fecal coliform bacteria or elevated levels of other parameters may not necessarily confirm the presence of domestic waste.
Optical brightener (OB) dyes are brightening agents which are currently added to most laundry detergents sold in the U.S. When exposed to certain wavelengths of ultraviolet (UV) light, OBs fluoresce in the visible region of the spectrum, making materials appear brighter. Water samples that similarly fluoresce when exposed to the same wavelengths of UV light are likely to contain detergent OBs. A variety of OB compounds are employed in laundry detergent formulations, in varying amounts. Because there are no natural sources for OBs, and because laundry effluent is a major component of both septic effluent and domestic wastes treated by OSTDS, OBs are very useful indicators of human waste in surface waters. The presence of OBs in water samples is highly indicative of contamination by sources of human pollution and generally indicates a relatively direct connection between an OSTDS, or some other waste treatment stream, and surface waters. The sensitivity and rapidity of fluorescence techniques make OB detection a cost-effective and useful surrogate for detecting human wastes.
Other substances in surface waters also fluoresce and can interfere with fluorescent OB detection. Natural fluorescent compounds are often present, particularly in freshwater originating as surface drainage from wetlands. Soluble organic components from soils and decomposing plant matter can give a tea- or coffee-color to bodies of water. The soluble organics may collectively be classified as humic acids, fulvic acids or tannins, and more generally may be termed colored, or chromophoric, dissolved organic matter (CDOM). Natural CDOM components are present in constantly changing concentration gradients in estuarine waterways, rivers, streams, lakes and other bodies of water. CDOM components fluoresce under UV light and can interfere with measurements designed to detect OBs.
Although both OB compounds and CDOM components fluoresce under UV light, each has specific and characteristic fluorescence signatures. OBs fluoresce when exposed to UV light at an optimal excitation wavelength in the very near-UV (300-400 nm) range, and exhibit emission wavelengths in the 400-480 nm range, with peak emission wavelengths near 440-450 nm. CDOMs, on the other hand, are a mixture of many different compounds and consequently their emission spectrum is much broader than that of optical brighteners. When exposed to similar UV light, CDOM components fluoresce over the entire range of wavelengths between 350-600 nm, and even beyond at wavelengths longer than 600 nm, resulting in a very broad signal spanning the entire wavelength range from 350 to 600 nm.